This invention relates to a contact charging member which electrically charges a member, an image forming unit including the contact charging member, and an electrophotographic image forming apparatus including the image forming unit, the apparatus including a printer, a copier, and the like.
A corona discharger has been widely used as an electric charger for uniformly charging an image carrier, for example a photosensitive drum of the image forming unit of the electrophotographic image forming apparatus. Recently, the corona discharger, however, is not widely used because it generates a lot of ozone harmful to a working environment. Instead of the corona discharger, a contact charging brush and a contact charging roller have come into use.
FIG. 7A shows a conventional charging brush 1 which is known from U.S. Pat. No. 5,294,962 and FIG. 7B shows a conventional charging roller 4 which is known from Japanese Patent No. 2,632,899.
The charging brush 1 is formed by sticking a base cloth 1b to a conductive base plate 1d with a conductive adhesive 1c, and the base cloth 1b has a large number of conductive bristles 1a planted thereon. A high-voltage power supply 2 applies a high voltage, for example about 1 kv, to the conductive base plate 1d and the conductive bristles 1a of the base cloth 1b bring their tips into contact with a cylindrical surface of a photosensitive drum 3 so that the surface of the drum is charged uniformly.
The charging roller 4 is formed by providing a conductive rubber layer 4b being relatively hard on a metal rod 4a and further providing a relatively high-resistance surface layer 4c on an outer surface of the rubber layer 4b. The metal rod 4a is applied with a high voltage by the high-voltage power supply 2, and a bottom end of a cylindrical surface of the charging roller 4 is pressed against the cylindrical surface of the drum 3 so that the cylindrical surface of the drum is charged uniformly.
According to the above described Japanese Patent, the hardness of the charging roller 4 is indicated by Asker C 54.5.degree., the surface layer 4c is formed by coating nylon resin to 10 .mu.m in thickness, and a volume resistivity of the charging roller 4 is 10.sup.9 to 10.sup.10 .OMEGA..multidot.cm.
The charging brush has such a problem that it tends to become dirty with dust or remaining toner attached on the surface of the photosensitive drum, and the attached dust or toner is hard to remove. Further, the dirty charging brush sometimes presents an unstable charging characteristic. For this reason, charging rollers have begun to attract the attention of those who work in this technical field.
In order to obtain a good image by using the charging roller, pressure applied to both ends of the metal rod must be uniformly distributed along the entire length of the rod so that the roller is pressed against the photosensitive drum uniformly. If there is a slight nonuniformity of charging on the drum, it causes a distinctive irregularity in a toner image formed on the photosensitive drum and lowers a quality of the image because a resolution of an image to be formed have been improved in recent years and toner particles have been made much finer.
In order to uniformly distribute the pressure applied to the both ends of the metal rod along its entire length in such a charging roller having a relatively hard conductive rubber layer, an outer diameter of the metal rod must be made large.
The largest image formed by a conventional desktop type image forming apparatus corresponds to a paper sheet of A4 size (210 mm.times.297 mm) defined by JIS (Japanese Industrial Standard) P0138. Recently, however, it is strongly demanded to form an image corresponding to a paper sheet of A3 size (two times as large as the A4 size) defined by JIS P0138, by the desktop type image forming apparatus. To meet this demand, the photosensitive drum must be made longer and consequently the charging roller must be larger. This makes the outer diameter of the metal rod and that of the charging roller larger, further makes the outer sizes of the image forming unit and those of the image forming apparatus larger. As a result of this, in a modern desktop type image forming apparatus wherein an image forming unit is rotatably separable in a vertical direction from the remaining members of the apparatus so that apparatus maintenance can be easily performed, it becomes difficult to open the image forming unit.
To prevent the outer dimension of the unit and those of the apparatus from becoming larger, the inventors of the present invention prepared such a small charging roller which had an outer diameter of 10 mm and a surface hardness of Asker C 45.degree., a metal rod of which had a diameter of 6 mm and a rubber layer of which had a thickness of 2 mm. And, the inventors operated an image forming apparatus provided with an image forming unit using the small charging roller. As a result of this operation, the small charging roller could not be substantially uniformly pressed against the photosensitive drum. Specifically, when a force pressing the charging roller against the circular surface of the photosensitive drum was increased, the charging roller bent to separate its longitudinal center portion from the surface of the photosensitive drum so that an image formed on a recording medium, for example a paper sheet, by the apparatus had a defect at. its portion corresponding to a longitudinal center portion of the surface of the drum from which the longitudinal center portion of the bent small charging roller was separated. When the force pressing the charging roller against the cylindrical surface of the photosensitive drum was decreased, the image formed on the sheet by the apparatus had many white spots.
This invention derived from the above described circumstances, and an object of the present invention is to provide a contact charging member which can not only charge a member uniformly but also be compact and has a long life time, an image forming unit including the contact charging member, and an electrophotographic image forming apparatus including the image forming unit.